Polyethylene resins are increasingly used for the manufacture of pipes. Thermal oxidative stability and environmental stress cracking resistance are two important measures to determine the long-term durability of pipes. Thermal oxidative stability can be measured by the oxidative induction time (OIT). The OIT is the amount of time the polymer or resin can be maintained in an oxygen atmosphere and at an elevated temperature before significant signs of oxidative degradation are observed. Environmental stress cracking is the formation of cracks in a material caused by relatively low tensile stress and environmental conditions. The environmental stress-cracking resistance (ESCR) test is usually performed by placing notched test specimens in a specified reagent under a load and recording the failure time of the specimens. The failure time is a measure of the ESCR.
While the thermal oxidative stability of polyethylene resins is usually improved by antioxidants, the environmental stress cracking resistance is often improved through the resin design. For instance, copending application Ser. No. 12/156,844, filed Jun. 5, 2008, discloses bimodal PE resins having improved cracking resistance. The ESCR value of the resin is increased by reducing the long-chain branching of the polyethylene. Co-pending application Ser. No. 12/380,519, filed Feb. 27, 2009, discloses polyolefin compositions having increased OIT. The polyolefin composition comprises a polyolefin, an acid and a primary antioxidant.
There is a continuing need in the industry for polyethylene resins having improved balance of properties suitable for pipe applications. There is a particular need for polyethylene pipe resins having both improved oxidative stability and environmental stress cracking resistance.